Decision theory (or the theory of choice; not to be confused with choice theory) is a branch of applied probability theory and analytic philosophy concerned with the theory of making decisions based on assigning probabilities to various factors and assigning numerical consequences to the outcome.

There are three branches of decision theory:

1. Normative decision theory: Concerned with the identification of optimal decisions, where optimality is often determined by considering an ideal decision-maker who is able to calculate with perfect accuracy and is in some sense fully rational.
2. Prescriptive decision theory: Concerned with describing observed behaviors through the use of conceptual models, under the assumption that those making the decisions are behaving under some consistent rules.
3. Descriptive decision theory: Analyzes how individuals actually make the decisions that they do.

Decision theory is closely related to the field of game theory and is an interdisciplinary topic, studied by economists, mathematicians, data scientists, psychologists, biologists, political and other social scientists, philosophers and computer scientists.

Empirical applications of this theory are usually done with the help of statistical and econometric methods.